The present invention relates to a reflector mirror, and more particularly, to a multi-layer back reflecting mirror having superior properties in terms of such aspects as freedom from glare, visibility and decorative finish.
The multi-layered back reflecting mirror of the present invention is useful in such applications as automotive rearview mirrors, mirrors with a convex surface that warn drivers against hazards on the road ahead, and ornamental mirrors.
Conventional mirrors such as automotive rearview mirrors are generally composed of a glass substrate that is coated on its back side with a metal coating such as of aluminum or chromium which is further overlaid with a protective coating.
Multi-layered surface reflecting mirrors have recently been marketed and they consist of a glass substrate that has a dielectric multi-layered coating on its front side and a black light-absorbing coating on the back side. Multi-layered back reflecting mirrors have also been proposed and they consist of a glass substrate that has a dielectric multi-layered coating on one side which is overlaid with a black light-absorbing coating. In the mirror of this type as disclosed in Unexamined Published Japanese Patent Application No. 144504/1982, the dielectric multi-layered coating consists of five alternate films of TiO.sub.2 and SiO.sub.2 that have thicknesses of .lambda./4 or 3.lambda./4 or a combination thereof. Another example of the multi-layered back reflecting mirror is disclosed in Unexamined Published Japanese Patent Application No. 98405/1985 and the dielectric multi-layered coating employed in this mirror consists of four alternate films of TiO.sub.2 and SiO.sub.2 each having a thickness of n.lambda./4 (where n is an odd number).
The above-described prior art reflector mirrors have spectral reflection characteristics as shown in FIG. 2, in which curve (a) refers to the aluminum mirror. As this curve (a) shows, the aluminum mirror has a high reflectance (80-90%) and flat reflection characteristics which are not dependent upon wavelength. If this aluminum mirror is used as a rear-view mirror in an automobile, it will cause strong reflection of the light flux coming from the headlights of a vehicle running behind at night and a driver who has adapted his sense of vision to the brightness of the field ahead will feel very tired because of the glare caused by this reflected light.
The spectral reflection characteristics of the prior art chromium mirror are shown by curve (b) in FIG. 2. As this curve (b) shows, the chromium mirror has a low reflectance (38-50%) and exhibits a certain degree of freedom from glare compared with the aluminum mirror having high reflectance. However, the glareless property of the chromium mirror is not as high as expected because of its flat spectral characteristics and, in addition, the visibility of the chromium mirror is rather poor.
A commercial multi-layered surface reflector mirror comprises a glass substrate that has a three-layered (TiO.sub.2 --SiO.sub.2 --TiO.sub.2) coating formed on the front side and a light-absorbing coating on the back side. This three-layered surface reflector mirror has spectral reflection characteristics as shown by curve (c) in FIG. 2. This reflector mirror has a reflectance of 48% and satisfies the reflectance requirement specified in JIS D 5705 "Automotive Mirror System" which calls for a minimum reflectance of 38% in the chromium mirror. However, as is clear from curve (c) in FIG. 2, the reflectance of this reflector mirror peaks in the range of 430-550 nm and drops sharply in the longer range up to 700 nm, causing the mirror to reflect bluish light. As a result, the color balance of the view provided by the mirror is greatly upset in such a way that a reddish color is particularly difficult to recognize and that the visibility of the mirror is impaired.
The spectral luminous efficiency V'(.lambda.) of the human eye, the spectral energy characteristics P(.lambda.) of a headlight of an automobile, and the product of P(.lambda.) x V'(.lambda.) are shown by three different curves in FIG. 3. As these curves show, the response of a driver's eyes to the headlights of a vehicle running behind at night is the highest at a wavelength between 480 and 550 nm. Since this range coincides with the region where the peak of the spectral reflectance curve (c) in FIG. 2 occurs, the ability of the three-layered surface reflecting mirror and other conventional surface reflecting mirrors to prevent glare from the headlights of a vehicle to the rear is not as high as expected. These surface reflecting mirrors have an additional disadvantage in terms of durability since the coatings formed on the glass substrate are highly prone to develop surface flaws when they are contacted by sand and dust or when they are rubbed by a brush during car washing.
The dielectric multi-layered back surface reflecting mirrors shown in Unexamined Published Japanese Patent Application Nos. 144504/1982 and 98405/1985 are formed by coating a dielectric multi-layered film on the back side of a glass substrate. Unlike the surface reflecting mirrors, these back surface reflecting mirrors have no problem in terms of durability but as is clear from curve (d) in FIG. 2 (the spectral reflectance curve of the mirror shown in Unexamined Published Japanese Patent Application No. 144505/1982) and from curve (e) (the spectral reflectance curve of the mirror shown in Unexamined Published Japanese Patent Application No. 98405/1985), the visibility of these back reflecting mirrors and their freedom from glare are not satisfactory for the same reasons as noted for the surface reflecting mirrors.